Heat exchanger for sheet glass drawing apparatus



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HEAT EXCHANGER FOR SHEET GLASS DRAWING APPARATUS June 14, 1966 6Sheets-Sheet 2 Filed April 7, 1960 FIG. 2

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HEAT EXCHANGER FOR SHEET GLASS DRAWING APPARATUS June 14, 1966 6Sheets-Sheet 4 Filed April 7. 1960 FIQ? FIG. 6

' INVENTOR. (EC/L 1Q. IVAQD June 14, 1966 c. R. WARD 3,

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FIGJO \24 IN VEN TOR. 628/4 2. Wflkfi June 14, 1966 c. R. WARD 3,256,082

HEAT EXCHANGER FOR SHEET GLASS DRAWING APPARATUS Filed April 7, 1960 6Sheets-Sheet a FIG. l 1

INVENTOR. Cit/L E. WAQD 'ATTOENE'Y 3,256,082 Patented June 14, 19663,256,082 HEAT EXCHANGER FOR SHEET GLASS DRAWING APPARATUS Cecil R.Ward, Gibsonia, Pa., assignor to Pittsburgh Plate Glass Company, acorporation of Pennsylvania Filed Apr. 7, 1960, Ser. No. 20,690 4Claims. (Cl. 65-204) This application is a continuation-in-part ofapplications Serial Nos. 807,915 and 828,836 filed April 21, 1959 andJuly 22, 1959, respectively.

This application relates to Heat Exchange Apparatus and particularly tothe employment of sets of special heat exchangers which substantiallybehave as black bodies in the thermal treatment of material,particularly in the form of a sheet or a ribbon. While the variousembodiments described herein relate to apparatus promoting area heatingor cooling of glass, it will be understood that the present invention isequally suit-able for use in the thermal treatment ofmaterials otherthan glass, such as metals, alloys, plastics, etc. In considering thedesign of heat exchange apparatus for use in the thermal treatment ofsheet material, such as glass, three characteristics are important. Thefirst characteristic is the uniformity of the radiant field produced bythe heat exchange apparatus. In present glass heating apparatus, heatingzones comprise coils mounted in generally rectangular or cylindricalchannels in insulating brick which forms the wall structure of the fur!nace. The temperature distribution over such an area is nonuniformbecause the channels containing the coils are maintained at aconsiderably higher temperature than the spaces between the channels.

' Even though a distance separates the heating elements from the path ofmovement of the sheet material to be heated or cooled, the work piece issubjected to a nonuniform radiant fluxpattern by virtue of thenon-uniform energy emittance over the area occupied by the heatingelements. This non-uniform radiant flux distribution makes the selectionof a control point to monitor a heatingsection verycri-ti-cal, becausedifferent points within the section are at different temperatures.Furthermore, different points within the non-uniform temperature area donot follow the eifective radiant level of the heating area.

A second characteristic required for a radiant heating assembly is itsefiiciency for heating the material. This is particularly critical inthe case of heating glass. When glass sheets are heated by heating coilsmounted in generally rectangular or cylindrical channels cut inrefractory material, the radiant energy output at any given temperatureis determined by the emissivity of the bricks or refractory materialused in the construction of the walls supporting the heating elements.This emissivity is usually considerably less than the emissivity of ablack body at that temperature. Because of this low efficiency, heatingcoils must be maintained at a higher temperature than is necessary whenthe coils are employed efiiciently in order to heat a work piece at adesired rate.

In heat exchange involving glass, the spectral distribution of theenergy emitted by the heating coils is shifted toward the visible regionwhere glass is transparent upon increasing the control temperature.Therefore, the eifect of heating the coils to higher temperatures doesnot necessarily result in an improved heating of the glass.

The third characteristic important for heating sections is the speed ofresponse of the heating elements to the requirement for different heatpatterns. Present heating furnaces are constructed of solid brickconstruction. The solid brick has a large thermal capacity necessitatingconsiderable time to change its temperature. On the one hand, much timeis required for a heat soaking at the start of any operation or betweenoperations when the new ope-ration involves a higher radiant level thanthe previous operation. Similarly, much time is consumed in cooling thesolid brick when a change in operation involving a lower radiant levelis desired. This slow response also makes it difficult to controlprecisely the radiant levels of heating sections. Operators cannotobtain the full advantage of recently developed precise control elementswhich respond very rapidly to variations in temperature from a desiredtemperature pattern.

The response time of glass lehrs and furnaces in use before the adventof the present invention was often so long that a change in thetemperature distribution over the heating area occur-red between thestart of an operation and later during its run. This resulted in a driftin the energy spectrum of the radiant field to which the glass wasexposed and it was often necessary to change the control temperatures ofthe lehr after the operation was in progress for a period of time tocompensate for such drift.

The lack of one or more of the three above characteristics combines toproduce .an additional effect which makes it almost'impossible for anytwo lehrs or furnaces or heat treatment apparatus of refractory materialto have identical operational performances. As a result, process datataken from one heating system cannot be utilized to provide a basis formaking or using another system without considerable experimentation andmodification.

The present invention in its broadest aspect covers the use of heatexchange apparatus for cooling as well as for heating. When a ribbon, asheet or other shape of material is to be controllably cooled, it isespecially impor-. tant that the cooler be as uniform and perfect a heatabsorber as possible or else the cooling rate cannot be controlledproperly or cannot be utilized at maximum etliciency, respectively.

Since the emissivity of a hot body relative to its surroundings is thesame as its absorptivity when it is colder than its surroundings, theterm emissivity as used in this disclosure covers both emissivity andabsorptivity.

The present invention avoids the above undesired detects by employingheat exchange apparatus comprising a series of refractory structuresincluding a plurality of smoothly surfaced walls of material having anemissivity of at least 50% and extending along parallel axes in planesoriented obliquely to each other, each wall terminating is sharplyangled relation to each adjacent wall to form a series of adjacentangular cavities of predetermined width and depth extending insideaby-side relationship, each cavity havingan acute apex angle.Additional walls are provided to form a hollow chamber with the surfaceopposite the wall surface forming the angular cavities. for solidrefractory minimizes the thermal capacity of the heat exchanger andpromotes rapid response when a change of radiant energy level isdesired.

A uniform radiant field needed for heating sheets of glass is producedby utilizing in a heating section of a furnace a number of adjacent heatexchangers of the type described above, each approximating a black bodycavity.

The heat exchangers are located to cover at least an.

tion containing 60% silica and the balance substantial- 1y clay isespecially elfective as a radiator, although pure silica andcompositions containing as little as 50% silica Substitution of thehollow chamber and 50% clay are effective. For radiant absorption, colddrawn steels have been employed successfully in the cooling of drawnsheet glass. However, other refractory materials may be employeddepending upon the temperatures of the ambient atmosphere involved inthe heat exchange operation.

The construction of the cavities, their depth, width and apex angles, iscorrelated with the minimum distance separating the radiant heatexchanger from the surface of the work piece to insure that the surfaceis exposed to a continuous, uniform field of black body radiation. Thislatter is accomplished by choosing such dimensions for the heatexchanger that a solid angle of black body radiation emanating from eachcavity overlaps that emanating from adjacent cavities at the surface ofthe work piece.

In order to insure proper control for the boundaries of the solid angleof black body radiation, the walls of the cavity must be smoothlysurfaced. Conventional furnace refractories are too rough and uneven attheir surfaces to control the internal reflections within the angularcavity as precisely as desired. Similarly, in cooling apparatus, aseries of members having approximate black body characteristics areconstructed with reference to a surface of a work piece to be cooled toprovide a uniform absorptivity.

In order to understand the present invention more completely, a numberof embodiments will be described illustrating how the present inventionmay be employed in certain illustrative heat exchange operationsinvolving glass ribbons or sheets.

In the drawings which form part of the present de- 'scription andwherein like reference numerals apply throughout, FIGURE '1 is aschematic ray diagram illustrating how the geometry of an angular cavityis determined.

FIGURE 2 is a chart showing how the emissivity of a cavity varies withthe emissivity of the material used to form the refractory structurewhere radiant energy is emitted with -no reflection, one reflection, tworeflections, three reflections and four reflections before reaching thetarget are-a. This chart is equally true for the absorption of radiantenergy impinging on the cavity and emanat ing from the target area.

FIGURE 3 shows how the emissivity or absorptivity of an angular cavityvaries with the ratio of depth of cavity to the width of its opening.

FIGURE 4 is an end view of one embodiment of heat exchanger employed asa heating element in a furnace, showing fragments of adjacent heatexchangers.

FIGURE 5 is a view similar to FIGURE 4 of an alternate embodimentheating element according to the present invention.

FIGURE 6 is a fragmentary view at right angles to FIGURE 5, disclosinghow individual heating elements are arranged in sets.

FIGURE 7 is a schematic view of a so-called horizontal f'urnaceemploying heating elements according to the present invention, whereinthe sets of heating elements are arranged in spaced horizontal planes.

FIGURE 8 is a schematic view of a so-called vertical furnace in whichglass sheets are supported in vertical planes :by ton-gs for heattreatment, wherein the radiant heaters constructed according to thepresent invention are \disposed on opposite walls of the heatingfurnace.

FIGURE 9 is a schematic view of a sheet glass drawing machine employingradiant heat exchangers or en- .ergy absorbers approximating black bodycharacteristics.

FIGURE 10 is a fragmentary elevation partly in section of a series ofradiant heat exchangers or energy absorbers included in the structuredepicted in FIGURE 9.

FIGURE 11 is a schematic of a sheet glass drawing .machine employingradiant heat exchangers or energy absorbers approximating black bodycharacteristics and showing a foot as the lower pass; and

FIGURE 12 is a fragmentary elevation partly in section of a series ofradiant heat exchangers or energy absorbers included in the structure ofFIGURE 11.

Referring to the drawings, the first three figures explain the criteriainvolved in determining how closely a cavity forming part of the heatexchange apparatus of the present invention conforms to a black bodyassembly.

FIGURE 1 depicts half the apex angle of an angular cavity formed betweensmoothly surfaced walls. R O P depicts a line running through a target,such as a surface of a glass sheet to be heated by a radiant heater orto be cooled by a black body absorber, and represents an areaintersected by a solid angle of black body radiation emanating from thecavity. L represents the depth of the angular cavity. w represents halfthe width of the cavity or half the base of an isosceles triangle formedby connecting the spaced ends of the two converging side walls formingthe cavity. d represents the distance between the base of the isoscelestriangle and the target surface.

If the emittance to or the adsorption from the target plane R O P is tobe an unchanging maxium, relatively insensitive to variations in thematerial of the cavity, then the cavity formed by the smoothly surfacedwalls must have an emissivity equal to that of a black body or unity.Since most materials have an emissivity less than one, it is necessaryto construct and shape the cavity in such a way as to utilize reflectiveradiation to augment the emitted radiation.

In the present case, radiation emitted from point A on the cavity topoint P in the target plane is composed of radiation emitted directlyfrom point A, that radiated from point B and reflected at point A, thatradiated from point C and reflected at points B and A toward point P,etc; If the emissivity of the material used for the walls of the cavityis at least 50%, the combination of emitted energy and the reflectedenergy approaches unity asymptotically.

FIGURE 2, which compares the emissivity of a cavity versus theemissivity of the material used to produce the cavity, discloses how theemissivity of a body approaches unity when utilizing multiplereflections. For a straight wall, the emissivity equals the emissivityof the material chosen. As the number of internal reflections isincreased before the energy is radiated from the cavity, it will be seenthat the emissivity of the cavity approaches unity very rapidly evenwith relatively low emissivity materials.

Where 98% efliciency is desired, FIGURE 2 discloses that material of 50%emissivity must be constructed to form an angular cavity providing atleast four internal reflections. With material of 70% emissivity, onlytwo internal reflections are needed to raise the emissivity of thecavity to 98%.

FIGURE 3 discloses how the emissivity of a cavity increases as the ratioof its depth to the width of its opening increases. This figureindicates that the smaller the apex angle of a cavity, the closer itapproaches black body characteristics.

FIGURE 1 indicates that as the emissive power of point A incorporates alarger number of reflective components that reinforce the beam frompoint A on the cavity to point P in the target area, point A approachesa black body radiator with respect to point P in the target plane. Theillustrated ray from point A to point P is an extreme ray in a solidangle of substantially black body radiation emanating from the cavity.The least possible number of reflected components is present in thisray. Thus, if the black body condition is obtained for the ray frompoint A to point P, then point A provides black body emittance to everyother point in the target plane R O P. It follows that every other pointin the cavity has maximum emittance to the target plane R O P.Therefore, the cavity is a black body emitter with respect to the targetplane. Likewise, if it is desired to use the heat exchange apparatus forcooling purposes, radiation emitted from a target plane R O P isabsorbed and the cavity behaves as a black body absorber.

In determining the apex angle of the cavity-for a given distance to atarget plane, it is necessary first to determine the number of reflectedcomponents required to raise the emissivity of the cavity to approachunity. This depends upon the emissivity of the material used.

In FIGURE 2, the emissivity of the cavity has been plotted against theemissivity of the refractory material for various numbers of reflectedcomponents reinforcing the directly emitted rays. As an example, if thecavity is constructed from a refractory having an emissivity of 0.80 andwith an angle such that at least twice reflected components are present,then the emissivity is at least 0.992.

Knowing the number of reflected component beams desired, the angle ofthe cavity required to give this number of reflections can bedetermined. Referring again to FIGURE 1, the angle a which is the angleof direct emission of radiation from point A to point P can bedetermined by the equation:

where w is the half width of the base of the cavity, h is the half widthof the target plane, d is the distance from the target to the cavitybase and x is half the apex triangle of the cavity.

Similarly, b, the angle of emission of radiation from point B whichwould add to the emissive power of point A by first order reflection, isgiven by:

and c, the angle of emission of radiation from point C which would addto the emissive power of point A by second order reflection, is givenby:

From these equations, the apex angle of the cavity can be determined forany refractory material with a given emissivity in order that sufficientreflected rays are present to bring the emissivity of the wedgesubstantially to unity. 98% or 99% is sufliciently good for commercialpurposes. The dimensions of the cavity can be obtained from therelationship (aretan Tangent x: w/ L where w is half the width of thebase of the cavity and L is the depth of the wedge.

In FIGURE 3 the curves shown are for a distance from cavity base totarget plane of 12 inches, wherein the target plane is 1.2 times as Wideas the base of the cavity. The curves are also valid for any cavity totarget distance greater than 12 inches. The target area has been chosenwider than the base of the cavity so that when the cavities are placedside by side, their respective black body fields will overlap into auniform pattern.

Use of present invention in radiant heating FIGURES 4 through 8 show theconstruction of individual heater elements and their arrangement invarious furnaces to provide heat for glass sheets.

A heat exchange member according to the present invention comprises ahollow refractory structure depicted generally by reference number 10 ofa material having an emissivity of at least 50%. Such member may beconstructed by slip casting a silica-clay material to insure that itssurfaces are smooth.

A typical procedure for slip casting hollow refractory structures 12inches long, 6 inches wide, 3 /8 inches high with grooves 1% inches wideand 1% inches deep formed of walls inch thick involves mixing 180 poundsof mesh fused silica grog with 120 pounds of ordinary clay and addingthe solid mixture to a solution containing 3,000 cc. Na P O in 54 poundsof distilled water to form a slip. The slip was poured into a plaster ofParis mold having inner walls shaped to the outer shape desired for therefractory structure. The slip solidified adjacent the walls of theplaster of Paris mold at the rate of 'inch thickness per 10 minutes.After 10 minutes, the excess slip was removed and the solidified slipwas premitted to air dry for about 10 minutes. The mold was then removedfrom the slip and the slip fired at 2156 F. for 72 hours.

Care must be taken to limit the firing temperature, because the fusedsilica changes into a high expansion form when it is fired tosubstantially higher temperatures. Also, the fused silica used must beof a fine mesh to promote smooth surfaces.

Each refractory structure 10 is constructed to provide a series ofsmoothly surfaced, longitudinally extending walls 12, which extend alongparallel axes longitudinally of the member 10 in planes orientedobliquely to each other where their smooth outer surfaces 14 formcavities 16 of V-shaped cross-section that extend in side-by-siderelationship along the length of the member 10.

Additional walls 18, 20, and 22 are attached to the outermost walls 12of the flanking cavities 16 to form a hollow chamber 24 with the innersurfaces 26 of the walls 12.

Walls 18 and 22 of each refractory structure are constructed to extendlinearly in parallel planes normal to the planes in which wall 20 isdisposed. Walls 18 and 22 are recessed adjacent the corners they formwith wall 20.. Part of each recess forms a groove 28 extendinglongitudinally of the refractory structure 10 along each wall 18 and 22.A clip 30 having a base 32 secured to the furnace structure terminatesin tongues 34 inserted into the grooves 28 to support the refractorystructure 10 to a furnace structure.

The refractory structures 10 are arranged in sets aligned longitudinallyand transversely of each other so that the series of refractorystructures 10 presents continuous lines of cavities 16 arranged inside-by-side relation. This is accomplished by actuating wall 22 of onerefractory structure 10 against Wall 18 of its neighbor.

FIGURES 5 and 6 disclose an alternate construction for the refractorystructures 10 in order to insure that the refractory structures 10 ofeach set are aligned properly in a longitudinal direction. This isaccomplished by threading rods 36 through the grooves 28 and shorteningwalls 20 and the recessed portions of walls 18 and 22 sufficiently toreceive apertured flanges 38 dependingfrom plates 40 secured to thefurnace structure. The apertures of the flanges 38 are located foralignment with grooves 28 when the refractory structures 10 are properlyplaced. Therefore, the rods 36 extend through the grooves 28 and alignedapertures of the apertured flanges 38 to insure proper alignment of therefractory structures 10 and their V-shaped cavities 16.

When the cavities 16 are employed as black body radiators, a source ofheat is required to be operatively associated with each refractorystructure 10. This source of heat may be provdied with passing heatedfluids, such as burning gases, through the hollow chamber 24 of therefractory structures 10. Since the walls 12 of the refactory structure10 are thin, preferably on the order of magnitude of about A; inch toabout inch in thickas possible within its cavity 16 and that it covers amaximum of about of the cross-sectional area of the aperture of saidcavity. If these precautions are not taken, the electrical resistanceheating element causes the cavity to lose its black bodycharacteristics.

In FIGURE 7, refractory structures 10 are employed in a horizontaltunnel-type furnace or lehr 44 having a roof 46, a floor 48 and walls50. Conveyor rolls 52 are rotatably mounted to the walls 50 and aredriven by conventional motor and drive means, such as chains andsprockets (not shown). Glass sheets or glass support means are movedthrough the furnace tunnel as the rolls 52 rotate. A set of refractorystructures 10 is attached to roof 46 and another set of refractorystructures 10 is attached to the floor 48 of furnace 44. Thermosensitivecontrol units 54 are mounted through the walls 50 and focused on areasof the emitting surfaces of the refractory structures 10 to monitor andcontrol the thermal output of the electrical resistance heating elements42 mounted in the apex of each cavity 16.

The electrical resistance elements are interconnected in suitableresistance circuits to 'lead wires 56, each of which is coupled to adifferent voltage source (not shown) through a control circuitresponsive to the reading supplied by a thermosensitive control unit 54.As many control circuits are provided along the roof and the fioor asare required to control the pattern of radiant heat both along andacross the path of glass travel through the furnace.

In horizontal furnace 44, a screen 58 of open mesh work configuration issupported above the lower set of heating elements to keep glassfragments from contacting the electrical resistance heating elements 42disposed below the conveyor in the event glass is broken during itspassage through the furnace. Otherwise, the resistance wires may becaused to burn out because of the presence of the glass fragments at thewires.

A vertical furnace 60 in which the refractory structures 10 are carriedby vertical 'walls 62 is shown in FIGURE 8. In this illustration ofanother structure employing the present invention, glass sheets G aregripped by tongs 64 carried by tong carriages 66. The latter aretransported through the furnace 60 by means of conveyor rolls 6S drivenby conventional driving means (not shown). Hot gases are passed throughthe hollow chambers 24 to provide a radiant heat source for the cavities16.

It is understood that either source of heat described may be employed ineither of the furnaces illustrated. Also, in either case, electricalresistances can be located within hollow chambers 24 to provide asuitable heat source for cavities 16.

While the apparatus depicted in FIGURES 7 and 8 is particularly usefulin heating glass sheets for soaking treatments such as required forannealing, tempering or coating, the refractory structures 10 areequally suitable for employment in lehrs for bending glass sheets. Whenglass sheets are bent in pairs preparatory to their lamination to formlaminated safety glass Windshields, only the upper surface of an assmblyof aligned Windshields is exposed to radiant heaters and masses of metalare dis: posed below selected portions of the assembly to withdraw heatfrom those portions that are to remain relatively flat. Hence, heatersare located only above the path of movement of the glass sheetassemblies. The refractory structures 10 of the present invention,therefore, may be disposed on either one side only of the path ofmovement taken by glass sheets or on both sides of the path of movement.

In order to verify the benefits of the present invention to provideuniform heating of a glass sheet, the following experiment wasperformed. A furnace 24 inches long, 18 inches high and 16 inches widewas first provided with solid refractory walls including refractorychannel member 1 /2 inches deep and 1%. inches wide of rectangularcross-section and separated from each other by 1 /2 inches to extendlongitudinally of the furnace in side-by-side relation to each otheralong the opposite walls of the furnace. Heating coils of /2 inchdiameter were carried in the channels and extended throughout theirength. After three hours of continuous heating, the temperature of theradiating surface of the solid walls varied between 1200 F. at theheating coils to 1110" F. intermediate the coils.

A glass sheet at room temperature and having dimensions inch thick and10 inches by 12 inches was suspended in a vertical plane at the middleof the furnace. Introducing the glass cooled the furnace. Enough currentwas supplied to the heating coils to cause their temperature to recoverto 1200 F. After 45 minutes during which time the furnace temperaturestabilized, the surface temperature of the glass sheet reached atemperature patern varying between 1100 F. and 1125 F.

The same experiment was performed after removing the solid refractorywalls and substituting cast sections of a silica-clay composition havingsmooth walls providing angular cavities 1% inch wide and 1% inch deepextending along the furnace walls in side-by-side relation for the solidrefractory walls. Heating coil of /2 inch diameter were installed withinthe cavities and heated to 1200 F. The surface temperature of thecavities varied from 1195 F. at their apex to 1185 F. at the widest partof the opening after only one hour of heating.

A sheet of glass as identical as possible in length, width, thicknessand chemical composition to the sheet heated by the furnace providedwith solid refractory walls was placed in the middle of the heatedfurnace provided with heat exchange members conforming to the presentinvention. The glass sheet reached a surface temperature that variedbetween 1185 F. and 1190 F. This temperature range was reached from roomtemperature (about 75 F.) in about 15 minutes.

A 10 kilowatt power supply was used to heat the coils in both of theexperiments described hereinabove. In other words, the same power inputcapacity was available for both furnace constructions.

From the results of these experiments, the response time of the emptyfurnace was reduced from 3 hours to 1 hour to heat the furnace to itsequilibrium conditions with the heating coils set at 1200 F., and from45 minutes to 15 minutes to heat a glass sheet to its equilibrium point.

Furthermore, the temperature gradient of the heater was reduced from F.to 15 F. and the glass surface temperature raised from a range of 85 F.to F. below the coil temperature to a range of 10 F. to 15 F. below thecoil temperature by employing heat exchangers according to the presentinvention rather than the solid refractory structures of the prior art.

Use of present invention in radiant heat absorption As statedpreviously, the present invention is equally susceptible to use in theconstruction of radiant heat absorbers. A typical example of such use isin the manufacture of sheet or Window glass.

In the manufacture of drawn.sheet glass, glass is drawn generallyupwardly in the form of a continuous ribbon from the surface of a bathof molten glass. The glass in its upward travel passes between variouscooling means. The conventional cooling means employed are usuallyconstructed of a refractory material, such as metal, usually in the formof a plurality of connected rectangular or square tubes for the passageof cooling fluid, such as water, therethrough and present a continuousplane surface to the glass. The high heat to which these conventionalcooling means are subjected causes a non-uniform-' 1y distributed scaleto form on their surfaces, thereby decreasing their heat absorbingefiiciency. Scaling of the cooling means is a particularly seriousproblem after they have been in use for some length of time. Thesecooling means also reflect heat back to the viscous glass, therebyfurther reducing their heat absorbing efliciency. The combination of thetwo described effects is a source of difiiculty in maintaining a uniformgauge or thickness of the sheet, and materially reduces the speed ofdrawing, so that a lesserquantity of glass is produced.

Various attempts have been made to increase the drawing speed byincreasing the size of the cooling means. However, as will be obvious,the problem of scaling and reflection of heat back to the glass stillexists. Also, attempts have been made to control gauge by varying theabsorbing properties of the cooling means, as by placing pads of heatresisting material, such as transite or asbestos, along the surface ofcooling means facing the glass. This requires a constant observation ofthe sheet and a constant changing of position of these various pads, andin addition, may discharge scale from the cooling means into the baththus contaminating the molten glass. The changing of pad position thusadds to the non-uniform scale problem and may score or mark the coolingmeans thus decreasing their useful life.

The present invention has been utilized to provide a maximum heattransfer from the sheet to the heat absorbers per unit absorber area, toprovide a uniform heat absorbing area resulting in a more uniformthickness of the sheet and to maintain a constant drawing speed over thekiln cycle, independent of uneven coating, scaling or marking of theheat absorbers.

Broadly, this aspect of the present invention includes the use of heatabsorbers made up of a series of jutaposed connected hollow membersmounted one in side-byside relation to another and interconnected,preferably in series, at their terminal ends, so that cooling fluid,such at Water, maybe fed therethrough. Each member has smoothly surfacedwalls extending along parallel axes in planes oriented obliquely to eachother and terminating in sharply angled relation to each adjacent wall.The juxtaposed walls of adjacent members form angular cavities ofpredetermined width and depth and extend in side-byside relationship.The assembly presents a surface of adjacent V-shaped cavities havingacute apex angles, facing the rising glass ribbon. The angle of eachcavity is such that any entering radiant energy from predetermined solidangles is more than 98% absorbed even though the absorptivity of thematerial of the heat absorbers is as low as 50%.

Turning now to 'FIGURES 9 and 11, there is shown a sheet of glass 100being drawn from a bath 102 of molten glass in a drawing kiln generallyindicated at 104. A draw bar 106 extending transversely of the kiln 104is submerged in the bath 102. The glass sheet 100 in its viscouscondition forms a base or meniscus 107 with the surface of the bath 102and is drawn from the bath 102 and through the drawing chamber 108 ofthe kiln 104 by means of drawing rolls 110 of a conventional drawingmachine generally indicated at 112. The drawing chamber 108 as depictedin the drawing, is defined by the bath 102, conventional L'blocks 114,ventilator water coolers 116, end walls 118 and catch pans 120. Theventilator coolers 116 are each positioned between an L block 114 andthe base framework of the drawing machine 112 and extend substantiallyto the end walls 118 of the kiln 108. The base of the drawing machine112 is substantially closed by means of the generally U-shaped catchpans 120, which are formed as coolers and are positioned so as to catchbroken glass which may drop in the machine and thus prevent entry offragments into the bath 102. These catch pans 120 also extendsubstantially to the end walls 118 of the kiln 108 and are constructedfor the passage of cooling fluid, such as water. One leg of each catchpan 120 is disposed substantially parallel to and spaced from the sheet100.

In the embodiment illustrated in FIGURES 9 and 10, heat absorbers 122constructed in accordance with the teachings of this invention areprovided for absorbing a maximum amount of radiant energy from each unitarea of the sheet I100. These heat absorbers 122 are spaced above thesurface of the bath 102 and are positioned on opposite sides of thesheet 100 to extend substantially the width of the sheet, transverselyof the kiln 108.

As depicted in the drawings, each heat absorber 122 is constructed ofjuxtaposed connected hollow members 124 of parallelogram section, havingsmoothly surfaced walls 126, 128 facing the surface of the ribbon ofglass.

100. The walls 126, 128 of each member extend along parallel axes inplanes oriented obliquely to each other and terminate in sharply angledrelationship, as at 130. The juxtaposed walls 126, 128 of adjacentmembers 124 form cavities 132 which extend in side-by-side relationship,each cavity having an acute apex angle 134. The cross section of themembers 124 may differ from that illustrated, so long as the cavities132 are as described.

The members 124 are series connected at their terminal ends for thepassage of a cooling fluid, such as water, therethrough, and to providefor the series connections and the passage of the cooling fluid manifoldboxes 136 are provided. Conduits 138 adapted to be connected to a sourceof cooling fluid and to' a sump (both of which are not shown) areconnected to the manifold boxes 136 for the inlet and outlet of coolingfluid to the members In the embodiment illustrated in FIGURES l1 and 12,heat absorbers 122' constructed in accordance with the teachings of thisinvention are provided for absorbing a maximum amount of radiant energyfrom each unit of the sheet 100. These heat absorbers 122' are spacedabove the surface of the bath 102 and are positioned on opposite sidesof the sheet to extend substantially the width of the sheet,transversely of the kiln 108. Each heat absorber 122 has a foot portionextending sub- 'stantially parallel to the surface of the bath andrearwardly of the sheet 100.

As depicted in the drawing, each heat absorber 122 is constructed of aplurality of juxtaposed connected hollow members 124 of parallelogramsection, having smooth- 1y surfaced walls 126, 128 facing the surface ofthe ribbon of glass 100. The walls 126, 128 of each member extend alongparallel axes in planes oriented obliquely to each other and terminatein sharply angled relationship, as at 130. The juxtaposed walls 126, 128of adjacent members 124' form cavities 132 which extend in side-by-siderelationship, each cavity having an acute apex angle 134. Thecross-section of the members 124 may differ from that illustrated, solong as the cavities 132 are as described.

The members 124 of the embodiment illustrated in FIGURES 11 and 12 areseries connected at their terminal ends for the passage of a coolingfluid, such as water, therethrough, and to provide for the seriesconnections and the passage of the cooling fluid manifold boxes 136 areprovided. Conduits 138 adapted to be connected to a source of coolingfluid and to a sump (both of which are not shown) are connected to themanifold boxes 136 for the inlet and outlet of cooling fluid to themembers 124.

Attached to the lower hollow member 124 and in series therewith is afoot member 140, shown as constructed of a plurality of side-by-sidehollow rectangular members 142. This foot presents a flat surface to therising glass ribbon and a flat surface to a portion of the bath 102,conditioning the bath adjacent the base of the sheet. As illustrated,the depth of the foot 140 from the sheet 100 toward the L-block islarger than the members 124.

The following experiments were performed with a drawing machine tocompare the effect of radiant heat absorbers constructed according tothe present invention with cooling means constructed according toteachings of prior sheet glass drawing art.

A drawing machine produced a ribbon of double strength glass (0.125 inchnominal thickness) at a given drawing speed using conventional, planefaced cooling means of a predetermined height positioned in a kiln ofthe construction shown in FIGURE 9. Increasing the height of theconventional, plane faced cooling means by approximately 30%, With otherconditions remaining constant, increased the drawing speed of themachine for the same thickness of glass by approximately 16%.

Using heat absorbers of approximately the same height as the first namedconventional plane faced cooling means and constructed in accordancewith FIGURES 9 and 10 of this invention, with other conditions stillremaining constant, resulted in a 17% increase in drawing speed for thedrawing machine producing a ribbon of the same width and thickness. Infact, its drawing speed with a shorter heat absorber was even greaterthan that produced using a higher cooling means of prior artconstruction.

Using heat absorbers of approximately the same height as in the exampleabove for the construction of FIG- URES 9 and 10 and constructed inFIGURES 11 and 12 of this invention, and with footed portions having ahorizontal dimension of 1.25 times the horizontal dimension of a member124, with other conditions remaining constant resulted in a 6% increasein drawing speed for the drawing machine producing a ribbon of the samewidth and thickness. This is a 24% increase in speed over that producedusing cnventional, plane faced cooling means with all other conditionsremaining constant and producing a sheet of the same width andthickness.

Generally, using a conventional drawing construction, the glass ribbonis allowed to vary :0.006 inch in thickness across a predetermined widthof ribbon of glass, and to maintain this thickness variation requireshourly or more frequent changing of positions of the aforementioned heatresisting pads. Using an arrangement, as shown in FIGURES 9 and 11, withother conditions remaining the same, it has been possible to hold theribbon to a thickness variation of less than half the above figureacross the same width ribbon with pads placed only over the manifolds136 and closely adjacent structure. Over a considerable period of time,such pads were not moved transversely of the heat absorber.

What is claimed is:

1. Heat exchange apparatus comprising an integral refractory structureincluding a plurality of smoothly surfaced walls of material having anemissivity of at least 50% and extending along parallel axes in planesoriented obliquely to each other, each wall terminating in sharplyangled relation to each adjacent wall to form a series of adjacentangular cavities of predetermined width and depth extending inside-by-side relationship, each cavity having an acute apex angle, andadditional walls attached to the outermost wall of the outermostcavities to form a hollow chamber on the side of said smoothly surfacedwalls opposite said angular cavities. A

2. A radiant heater comprising a refractory structure including aplurality of smoothly surfaced walls of a refractory material consistingessentially of a silica-clay composition containing at least 50% silicaand the balance substantially entirely clay having an emissivity of atleast 50%, and extending along parallel axes in planes orientedobliquely to each other, each wall terminating in sharply angledrelation to each adjacent wall to form a series of adjacent angularcavities of predetermined width and depth extending in side-by-siderelationship, each cavity having an acute apex angle, a radiant heatsource cooperating with said radiant heater, and additional wallsattached to the outermost wall of the outermost cavities to form ahollow chamber on the side of said smoothly surfaced walls opposite saidangular cavities.

3. Heat exchange apparatus comprising a refractory structure including aplurality of juxtaposed connected hollow members having smoothlysurfaced walls and of a material having an absorptivity of at least 50%,said walls of each member extending along parallel axes in planesoriented obliquely to each other and terminating in sharply angledrelation to each adjacent wall, the juxtaposed walls of adjacent membersforming angular cavities of predetermined width and depth and extendingin side-by-side relationship, each cavity having an acute apex angle,and means to allow an inlet and an outlet for the passage of. coolingfluid to and from said hollow members.

4. Apparatus for improving the speed of manufacture of drawn sheet glassdrawn generally upwardly from a bath of molten glass during a continueddrawing cycle comprising a pair of radiant heat absorbing cooling means,each positioned adjacent one surface of the glass sheet and extendingthe width of the sheet, each cooling means including an assembly ofhollow members of parallelogram section and joined together to presentopenended angular cavities of predetermined width and depth facing thesurface of the sheet, each of said cavities having walls defining anacute apex angle therebetween, each assembly having a substantiallyrectangular foot joined at its lower terminus and presentingsubstantially plane surfaces facing the surface of the sheet and thebath of molten glass, each foot including an assembly of hollow membersof rectangular section, said last-named assembly extending from thesheet a greater distance than said first-named assembly, all said hollowmembers being connected to one another for the continuous passage ofcooling fluid therethrough, and means to allow the flow of cooling fluidto and from said connected hollow members.

References Cited by the Examiner UNITED STATES PATENTS 1,545,893 7/1925Gregory 165115 1,584,241 5/1926 Mulholland 263-8 1,762,201 6/1930 Strong1320 2,151,983 3/1939 Merrill 60 2,167,333 7/1939 Foss -75 2,176,99910/1939 Miller 65-107 2,352,539 6/1944 Halbach 6584 2,607,168 8/1952Drake 65-204 2,794,300 6/1957 Golightly 65-158 2,828,948 4/1958 Caldwellet al. 165104 FOREIGN PATENTS 87,927 8/1956 Norway.

OTHER REFERENCES Handbook of Chemistry and Physics, 34th edition, page2512.

DONALL H. SYLVESTER, Primary Examiner.

CHARLES R. HODGES, Examiner.

C. I. LAICHE, D. CRUPAIN, Assistant Examiners.

1. HEAT EXCHANGE APPARATUS COMPRISING AN INTEGRAL REFRACTORY STRUCTUREINCLUDING A PLURALITY OF SMOOTHLY SURFACED WALLS OF MATERIAL HAVING ANEMISSIVITY OF AT LEAST 50% AND EXTENDING ALONG PARALLEL AXES IN PLANESORIENTED OBLIQUELY TO EACH OTHER, EACH WALL TERMINATING IN SHARPLYANGLED RELATION TO EACH ADJACENT WALL TO FORM A SERIES OF ADJACENTANGULAR CAVITIES OF PREDETERMINED WIDTH AND DEPTH EXTENDING INSIDE-BY-SIDE RELATIONSHIP, EACH CAVITY HAVING AN ACUTE APEX ANGLE, ANDADDITIONAL WALLS ATTACHED TO THE OUTERMOST WALL OF THE OUTERMOSTCAVITIES TO FORM A HOLLOW CHAMBER ON THE SIDE OF SAID SMOOTHLY SURFACEDWALLS OPPOSITE SAID ANGULAR CAVITIES.
 4. APPARATUS FOR IMPROVING THESPEED OF MANUFACTURE OF DRAWN SHEET GLASS DRAWN GENERALLY UPWARDLY FROMA BATH OF MOLTEN GLASS DURING A CONTINUED DRAWING CYCLE COMPRISING APAIR OF RADIANT HEAT ABSORBING COOLING MEANS, EACH POSITIONED ADJACENTONE SURFACE OF THE GLASS SHEET AND EXTENDING THE WIDTH OF THE SHEET,EACH COOLING MEANS INCLUDING AN ASSEMBLY OF HOLLOW MEMBERS OFPARALLELOGRAM SECTION AND JOINED TOGETHER TO PRESENT OPENENDED ANGULARCAVITIES OF PREDETERMINED WIDTH AND DEPTH FACING THE SURFACE OF THESHEET, EACH OF SAID CAVITIES HAVING WALLS DEFINING AN ACUTE APEX ANGLETHEREBETWEEN, EACH ASSEMBLY HAVING A SUBSTANTIALLY RECTANGULAR FOOTJOINED AT ITS LOWER TERMINUS AND PRESENTING SUBSTANTIALLY PLANE SURFACESFACING THE SURFACE OF THE SHEET AND THE BATH OF MOLTEN GLASS, EACH FOOTINCLUDING AN ASSEMBLY OF HOLLOW MEMBERS OF RECTANGULAR SECTION, SAIDLAST-NAMED ASSEMBLY EXTENDING FROM THE SHEET A GREATER DISTANCE THANSAID FIRST-NAMED ASSEMBLY, ALL SAID HOLLOW MEMBERS BEING CONNECTED TOONE ANOTHER FOR THE CONTINUOUS PASSAGE OF COOLING FLUID THERETHROUGH,AND MEANS TO ALLOW THE FLOW OF COOLING FLUID TO AND FROM SAID CONNECTEDHOLLOW MEMBERS.